A pertinent conventional rotary impact tool is disclosed in Japanese Laid-Open Patent Publication No. 2001-260042 (Japanese Patent No. 3670189).
The rotary impact tool disclosed in this publication is an impact driver, which is configured to allow setting of the number of times that the hammer apply impacts to the anvil so that a number of screws or the like can be tightened with the same torque. More specifically, the impact driver has a piezoelectric buzzer detecting the impact sound of the hammer on the anvil, a setting dial for setting the number of impacts, and a motor control unit. And, at a stage where impacts have been applied by a set number of times during the tightening of screws, the motor control unit stops the motor. This enables a number of screws or the like to be tightened with the same torque.
However, If the kind of screws and the material, thickness, dimension, etc. of a plate material to which the screws are tightened are changed, it is necessary to change the tightening torque, and therefore, each time they are change, the number of impacts must be reset.
As shown in FIG. 5, in the case that a tex screw (registered trademark) 3, whose front end portion is formed as a drill gimlet, is used, holes are to be formed in plate materials 4 and 5, so that it is necessary to rotate the end tool of the impact driver at high speed. As a result, the interval between the impacts after seating of the tex screw 3 is very short. Thus, it is difficult to set a proper number of impacts; further, since the rotation of the hammer is at high speed, the impact force is also increased. This may lead to decapitation or the like, in which the head of the tex screw 3 is torn off.
Further, in the case that the tightening completing timing (motor stopping timing) is determined based on the judgment by the operator regardless of the number of impacts, it is difficult to determine the tightening completing timing if the interval between the impacts is very short, and unintended impacts are applied, decapitation or the like, in which the head of the tex screw 3 is torn off, is likely to be caused.
Therefore, there is a need in the art to reduce the impact force and to make the interval between impacts relatively long, thereby preventing decapitation or the like of a screw, even in the event that it is necessary to rotate a screw or the like at high speed.